1. Field of the Invention
The present invention relates to a method and system for controlling ultrasound scanning sequence so as to permit detection of a low flow velocity.
2. Description of the Related Art
Conventionally, an apparatus has been used which combines the ultrasonic Doppler method and the pulse echo method to acquire a blood-flow image and a tomogram image (B-mode image) of a subject under examination and displays the acquired images in colors in real time. The principle of measurement of blood-flow velocity used in the apparatus will be described hereinafter.
When ultrasound beams are transmitted to blood flowing within the living body of a subject, the beams are scattered by moving blood cells so that the center frequency fc of the beams is Doppler-shifted by a Doppler frequency fd. As a result, the received ultrasound frequency f becomes f=fc+fd. In this case, the Doppler frequency fd is represented by EQU fd=2v cos .theta..times.fc/c . . . (1)
where v is a blood flow velocity, .theta. is an angle made by the ultrasonic beam with the blood vessel, and c is the acoustic velocity. It will thus be understood that the Doppler frequency fd is used to detect the blood flow velocity v in this way.
The blood flow velocity v is displayed in a two-dimensional form as follows. First, as shown in FIGS. 1 and 2, ultrasound probe 1 transmits ultrasound beams sequentially to the subject in directions of A, B, C, . . . by pulse signals provided from transmitting circuit 7 under the sector-scan control. In place of the sector-scan control the linear-scan control may be performed.
For example, echo signals of the ultrasound beams transmitted in the direction of A, which has been Doppler-shifted by the blood flow, are received by ultrasonic probe 1 and applied to receiving circuit 2 after conversion of an electric signal. Phase detecting circuit 3 detects Doppler signals from the received echo signals. The Doppler signals at, for example, 256 sampling points along a scan line (in the direction A) of the ultrasound beam transmitted are detected. The Doppler signals detected at each sampling point are frequency-analyzed in frequency analyzer 4 and then provided to display 6 via digital scan converter (DSC) 5. As a result, a blood-flow-velocity distribution image in the direction of A is displayed in real time.
Subsequently, the same operations are repeated for each of the scan directions of B, C, . . . , and blood-flow-velocity distribution images for scan directions are displayed as a two-dimensional image.
It is to be noted that the detectability of a blood flow velocity depends upon the data length of a Doppler signal. That is to say, if the sampling frequency of the Doppler signal is fr and the number of samples is n, then the data length T of the Doppler signal will be given by EQU T=n/fr . . . (2)
In this case, the frequency resolution .DELTA.fd will become EQU .DELTA.fd=1/T . . . (3)
Therefore, the Doppler frequency fd min corresponding to the measurable lower limit of the flow velocity will be represented by EQU fd min=1/T=fr/n . . . (4)
Therefore, it will be understood that either the sampling frequency fr of the Doppler signal has to be lowered, or the sampling number n has only to be increased (see FIGS. 3 and 4, FIG. 4 is obtained by fourier-transforming the waveform shown in FIG. 3), in order to detect the blood flow of a low velocity.
In the two-dimensional Doppler blood-flow imaging, the next relationship holds. EQU Fn.times.n.times.m.times.(1/fr')=1 . . . (5)
where Fn is the number of frames displayed in one second, m is the number of scan lines and fr' is the repetition frequency of the ultrasound pulses. The frame number Fn is associated with the real-time display of a two-dimensional blood flow image and usually takes a value between 8.about.30 so that 8.about.30 frames will be displayed in one second. For example, in the sector scan as shown in FIG. 5, when the scan line number m=32, the repetition frequency fr' of ultrasound pulses=4 KHz, and the sampling number n=8, the frame number Fn will become about 16. That is, if the scan line number m increases, the frame number Fn decreases, causing flicker. If the scan line number decreases, the density of scan lines would become coarse and hence the quality of image would be degraded.
Therefore, an apparatus has been desired which is capable of detecting a low flow velocity without decreasing the number of frames and degrading the image quality.